This invention relates to improvements in superconducting wires, and in particular to a method for making niobium-tin alloy wire.
Niobium-tin alloy superconductors are brittle and can be easily damaged, especially when drawn into fine filamentary wire and wound for use in electromagnets. The superconducting material in niobium-tin based superconductors is the intermetallic compound triniobium tin, or Nb.sub.3 Sn. The Nb.sub.3 Sn is the brittle material in the wire and the continuity of Nb.sub.3 Sn lamina extending the length of the wire must be protected to maintain the continuous superconducting path within the wire. As a result, wire production is accomplished using the pure metals niobium and tin with heat treatment after wire drawing to react the niobium with the tin to form a continuous layer of Nb.sub.3 Sn on the niobium. As used herein a niobium-tin wire is a wire surrounded by a copper sheath which acts as a stabilizer and contains at least niobium and tin. Small amounts of other elements, such as titanium, may be added to improve the current carrying properties of the superconducting Nb.sub.3 Sn compound formed after heat treatment. The stabilizer that surrounds the wire helps stabilize some of the wires functions, for example by acting as an alternative current path when the wire changes from superconducting to non-superconducting, and mechanically strengthening the wire.
Heretofore niobium-tin wires have been made by providing a copper sheathed wire containing niobium metal in the form of rods or expanded sheet, and tin in the form of a tin alloy such as bronze or as tin rods surrounded by copper. The niobium and tin are in close proximity so that during subsequent heat treatment tin reacts with niobium to form continuous laminae of Nb.sub.3 Sn on the niobium. The process of alloying the tin and niobium to form Nb.sub.3 Sn is hereinafter referred to as reaction-forming or reaction annealing.
Before reaction annealing, the wire is ductile and can be easily wound around a mandrel to form a coil. During annealing, the brittle intermetallic compound Nb.sub.3 Sn forms within the wire and thereafter the wire can only be wound into a coil by using special precautions. During the annealing process to reaction-form Nb.sub.3 Sn, many difficulties are experienced that detrimentally affect the superconducting properties of the wire or its ability to be processed. For example, the insulation surrounding the copper sheath can be easily damaged in the annealing process causing the wire to shift or even fuse together. In superconducting magnets formed from the wire if the wire shifts excessively during the anneal, the desired wrap positions are not maintained. When wire wrap positions are not maintained, the desired magnetic field homogeneity is not obtained during operation of the magnet. If wires fuse together or even touch, an electrical short between turns develops. Such shorts make it very difficult to continuously increase the magnetic field in the superconducting magnet when it begins operating. If it is desired to further process the wire after annealing, fused wires make it impossible to unwind the coiled wire without damaging it. Sometimes, hereafter an electrical short is more simply referred to as a short.
Two general prior art methods for reaction-forming Nb.sub.3 Sn in a coil form are known. In one method, called the "react and wind" process, the insulated, copper sheathed niobium-tin wire is wound onto a spool and annealed to reaction-form Nb.sub.3 Sn. Then it is carefully rewound onto a coil form for the superconducting magnet. The anneal is a series of heat treatments ranging from about 200.degree. C. to 700.degree. C. that may take from one to two weeks. The anneal treatments are controlled to react the niobium and tin to form a superconducting Nb.sub.3 Sn that will meet the designed current carrying properties required in a superconducting device such as a magnet. If wires fuse together during the anneal, the spool cannot be unwound without damaging the wire and degrading the superconducting properties of the wires.
In a second process called the "wind and react" process, the insulated, copper sheathed, niobium-tin wire is wound directly onto the coil form of the magnet. The wire is then reaction annealed as explained previously in a series of treatments to form the superconducting Nb.sub.3 Sn. A disadvantage to this method is that the coil form must be made from a material that can withstand the annealing treatments. For example, a fiberglass-resin coil form could not withstand the annealing temperatures. In addition, the magnet formed by this method is very susceptible to the fusing, shorting, and wire shifting problems mentioned previously.
Niobium-tin wire is susceptible to damage in both the wind and react, and the react and wind processes from still other sources in the annealing process. Because tin melts at approximately 230.degree. C. it is molten at the temperatures required to reaction-form the Nb.sub.3 Sn wire. Therefore, any punctures or defects in the copper sheathing of the wire may result in leakage of the molten tin. Such defects or punctures can be caused by the wire drawing methods or inclusions in the copper sheath. Tin leakage damages the insulation, creates resistive or normal current conducting regions in the wire, and when leakage is extensive essentially no Nb.sub.3 Sn forms. Stresses placed on the wire during annealing have been found to increase the tendency for tin leakage at such inclusion defects or cracks, and for damaging the superconducting path of Nb.sub.3 Sn.
What we have discovered and found as a contributing cause of the problems recited above is that niobium-tin wire contracts in length during the reaction annealing necessary to form Nb.sub.3 Sn. Most of the length contraction in the niobium-tin wire can be achieved before the brittle intermetallic Nb.sub.3 Sn forms. This is surprising since it is well known that the major constituents of this wire; copper, niobium, and tin or a tin alloy, have positive coefficients of thermal expansion and therefore would be expected to expand and cause the wire to increase in length when heated at temperatures up to 700.degree. C.
Since the niobium-tin wire in prior art annealing was tightly coiled around a solid mandrel, the length contraction caused the wire and its surrounding insulation to develop internal tensile stresses. This occurs because the coiled wire is constrained from contracting on the solid mandrel during the reaction anneal. These stresses can combine with any imperfections, inclusions, or defects in the wire or surrounding insulation to exacerbate punctures, cracks or breaks in the wire and its surrounding insulation. It is important to note that such internal tensile stresses can easily damage the continuous laminae of brittle Nb.sub.3 Sn that are formed within the wire and extend the length of the wire. As a result, such punctures, cracks or breaks can partially or completely disrupt the superconducting properties of the wire. In addition, during the reaction anneal such punctures, cracks or breaks in the wire will allow molten tin leakage while cracks or breaks in the insulation may allow separate turns of wire to touch causing fusing and shorting of the wire. The wire contraction alone or in combination with damage to the insulation may also cause substantial shifting of the wire from calculated wrap positions desired for optimum performance of a superconducting magnet.